An oil rig is a unit that enables the exploitation of hydrocarbon fields at sea, in other words the extraction, production or storage of oil and/or other gases such as, by way of non-limiting example, hydrocarbons, said products being located offshore sometimes at very great depths.
There are two types of rigs for the exploitation of offshore hydrocarbon and/or gas deposits:                firstly, fixed rigs that rest on the seabed and can thus be rigidly connected to oil wellheads and subsea pipelines;        then, Floating Production, Storage and Offloading units (hereinafter referred to as FPSO).        
By way of a preferred but non-limiting example, a rotating joint will be considered in its application within an FPSO unit.
Such a floating unit generally takes the form of a ship moored to the seabed by a fixed or disconnectable system that enables, depending on environmental conditions, the rotation of floating supports about a central mooring point, in principle a mooring turret.
Various devices are present on said floating supports that make it possible:                to process the hydrocarbons coming from a subsea reservoir and to separate the oil from the other components such as, by way of non-limiting examples, gas, water and sand;        to store oil and/or other gases in advance so as to be able, subsequently, to export them using shuttle tankers;        to re-inject into the reservoir the water and/or gas extracted from the oil and/or other gases that cannot be stored on the rig;        to inject into the oil wellheads chemical products used to protect the said wellheads against corrosion and the formation of various by-products capable of disrupting the operation of said wells;        to control the subsea installations by means of hydraulic and/or electrical commands.        
The mooring turret is connected to the floating support by a bearing system, said bearing system allowing the vessel to rotate about the fixed geostatic part of the turret, said turret being attached to the anchoring system. A turret can advantageously be located internally or externally with respect to the vessel, its position depending chiefly on the structure of the hull and the number of flexible lines connected to the turret.
Furthermore, on the fixed part of the mooring system, a fluid transfer system allows the connection of subsea pipelines to the floating production unit. In fact, inside the turret, a rotating joint or an assembly of rotating joints (also known as a swivel joint or a swivel stack) allows a transfer of fluid to be made between the geostatic part and the free part of the vessel that rotates about the turret.
The rotating joints thus ensure that all of the fluids, be they gaseous or liquid, are safely transferred from the geostatic parts such as, by way of non-limiting examples, oil wells, pipelines submerged beneath the seabed, manifolds and hoses to the system enabling these movements. There are two main types of rotating joints:                pipe-swivel also known as “in-line swivel joints”        toroidal-swivel joints.        
Pipe-swivel joints are the simplest fluid transfer systems. They contain a single fluid passage. When more than one fluid passage is required, toroidal-swivel joints are used: due to their large diameter, it is possible to provide a large number of fluid passages by stacking and/or assembling several toroidal-swivel joints together.
An example of a “conventional” toroidal-swivel joint comprises a certain number of main components. It comprises a male member, also called an “internal member,” and a female member, also called an “external member,” movable in relation to each other and kept concentric and coaxial with the aid of a mechanical bearing, by way of a non-limiting example a bearing with three rollers (also known as a 3-race roller bearing). In principle, the mechanical bearing enables the positioning, transmission of stress and rotation between the male and female members by replacing sliding with a bearing. In fact, the power absorbed by the resistance to rolling is much lower than the power absorbed by the resistance to sliding. The choice of a specific bearing is made on the basis of the dimensions and loads that the said bearing must withstand. A bearing usually comprises two rings, one internal and the other external, with integrated races. Rolling elements are arranged between the rings in the raceways. Rolling elements are, depending on circumstances, balls, cylindrical rollers, needles or tapered or spherical rollers. A cage usually guides the rolling elements, keeps them an equal distance apart and prevents them from coming into contact with each other. There are therefore four main types of bearings:                ball bearings;        tapered roller bearings;        cylindrical roller bearings;        needle bearings.        
Usually, a 3-race bearing is used in rotating joints. This bearing has three rows of bearings to move in two directions: two rows of radial rollers and one row of axial rollers.
A toroidal chamber is formed between the male and female members, forming an enclosed space. It is through this chamber that the fluid is transferred. A plurality of chambers may be present within the same rotating joint in order to ensure the passage or transfer of one or more fluids.
In order to ensure sealing within a rotating joint, one or more seals can be arranged on each side of the toroidal chamber, enabling the formation of a narrow fluid passage. The seals are positioned respectively within grooves provided for this purpose. The grooves may advantageously but not necessarily result from toroidal cavities, made on the surface in the internal wall of one or other of the male and female members. The said cavities can be machined or obtained directly by molding the female or male member of the rotating joint. In principle, any type of sealing ring can be used to ensure sealing within a rotating joint. The sealing required is dynamic sealing since the male and female members rotate relative to each other. By way of non-limiting examples, the seals used can be: O-rings and U-shaped lip seals with one or more flexible lips. Furthermore, a seal is advantageously made of a material that is compatible with the transferred fluid or fluids to be sealed, resistant to anti-extrusion clearances. Advantageously but in no way limiting, the seal can be made of a synthetic elastomer such as polytetrafluoroethylene (PTFE) or silicones.
In particular, in the offshore field, the design of rotating joints is very often subject to high-pressure and high-temperature requirements. On occasion, the large size of the device can also have an impact on the design of a rotating joint. In current practice within the offshore field, two types of toroidal-swivel joints are usually used: the piston seal toroidal-swivel joint and the face seal toroidal-swivel joint. They will be described in relation to FIGS. 1 and 2.
FIG. 1 shows a first known embodiment of a toroidal-swivel joint, known as a piston seal swivel joint. This is the simplest and most conventional embodiment.
A rotating joint 11 comprises a male member 12 and a female member 13, kept concentric with the aid of a special mechanical bearing 16, a bearing that has three cylindrical rollers. The male 12 and female 13 members are movable in relation to each other. Said bearing 16, positioned above the male 12 and female 13 members, has numerous advantages: it has high resistance to shocks, it withstands very high radial loads and is suitable for high rotation frequencies. A toroidal chamber 15, to guarantee the transfer of a fluid, is formed between the male 12 and female 13 members, the fixed and rotating members respectively. In order to ensure the seal of the toroidal chamber 15, three seals 14a, 14b, and 14c are present: they are advantageously installed within grooves, more precisely cavities 7a, 7b and 7c made in the internal wall in the female member 13. As a variation, the cavities can be located in the male member 12. The cavities are made, advantageously but not in a limiting way, by machining with a machine tool using a boring or milling process or obtained by molding the female member 13. The seals 14a, 14b, and 14c are preferably, but not limited to, O-rings or U-shaped double lip seals. Said seals comprise a heel cooperating with two flexible lips. In this first particular embodiment, the lips of seal 14a, 14b, and 14c project parallel to the axis of revolution of the said seal. Seal 14a, 14b, and 14c guarantees sealing in the following manner: the two lips keep seal 14a, 14b, and 14c in contact with the cavity 7a, 7b and 7c which accommodates the said seal, defining the surface to be sealed, and thus ensure sealing. The lips follow the profile and shape of the cavity of the rotating joint 11. The heel enables the lips to cooperate and be held with the rest of the seal: the said heel and lips form a single entity. Sealing is ensured thanks to the lips, each lip maintaining contact on a surface 12a, 12b or 12c respectively of the fixed male member 12 and on a surface 13a, 13b or 13c of the rotating female member 13. Similarly, when the seal is an O-ring, sealing is ensured by the said O-ring on two contact surfaces: one 12a, 12b or 12c on the fixed male member 12 and the other 13a, 13b or 13c on the rotating female member 13.
Nevertheless, this configuration has a certain number of drawbacks. Over time, the seals, whatever their type, are subject to two classes of related deformations: extrusion and creep. Creep can be defined as a slow and delayed deformation of a body subject to a constant stress, caused by the period of application of this stress. In a rotating joint, the seals are subject to repeated movement, namely rotation, which corresponds to the said constant stress, and leads, over time, to a deformation of the said seals. In a standard assembly, the extrusion clearance of the seal increases with pressure and with the diameter of the seal. Furthermore, in the context of preventing the failure of the seal due to the extrusion phenomenon, the greater the pressure, the smaller the extrusion clearance permitted by the seal. Consequently, the implementation of this first embodiment is no longer appropriate when the pressure of the device and/or the diameter of the rotating joint increase. Moreover, sealing within the rotating joint is assured by the contact of each lip of the seal with one surface of the male and female members of the rotating joint. The two male and female members being in rotation relative to each other, the seal undergoes a shearing phenomenon, which could result in the seal failing to fulfill its role.
FIG. 2 shows a second known embodiment of a toroidal-swivel joint known as a face seal swivel joint.
As in the first embodiment described above, a rotating joint 21 comprises a male member 22 and a female member 23, kept concentric with the aid of a special mechanical bearing 26, a bearing that has three cylindrical rollers. The male 22 and female 23 members are movable in relation to each other. The said bearing 26, positioned above the male 22 and female 23 members, has numerous advantages: it has a high resistance to shocks, it withstands very high radial loads and is suitable for high rotation frequencies. A toroidal chamber 25 is formed between the male 22 and female 23 members, the rotating and fixed members respectively.
In order to ensure sealing of the toroidal chamber 25, three seals 24a, 24b and 24c are present: in this particular embodiment, as described above, the said seals 24a, 24b and 24c are advantageously positioned or installed within grooves, more precisely cavities 27a, 27b and 27c made in the female member 23. The cavities 27a, 27b and 27c are made, advantageously but not in a limiting way, by machining with a machine tool using a boring or milling process or obtained by molding the cavity directly in the female member 23. The female 23 and male 22 members, however, have a particular shape adapted to prevent any deformation of the seals 24a, 24b and 24c due to mechanical axial clearances. In fact, one or more annular protuberances are present on the male member 22. The said protuberance or protuberances are inserted in one or more grooves made in the female member 23 to accommodate the said male member. In order to accommodate seals 24a, 24b and 24c, cavities 27a, 27b and 27c are advantageously arranged on the internal wall of the female member within the space provided to accommodate the protuberance or protuberances. Furthermore, seals 24a, 24b and 24c are preferably, but not limited to, O-rings or U-shaped double lip seals, said seals comprising a heel cooperating with two flexible lips. In this second particular embodiment, the lips of the seal project in a plane normal to the axis of revolution of the said seal. The seal guarantees sealing in the following manner: the two lips keep the seal in contact with the cavity, defining the surface to be sealed, and thus ensure sealing. The lips follow the profile and shape of the cavity of the rotating joint 21. The heel enables the lips to cooperate and be held with the rest of the seal: the said heel and the lips form a single entity. Sealing is ensured thanks to the lips, each lip maintaining contact on a surface 22a, 22b or 22c, respectively, of the rotating male member and on a surface 23a, 23b or 23c of the fixed female member 23. Similarly, when the seal is an O-ring, sealing is ensured by the said O-ring on two contact surfaces: one 22a, 22b or 22c on the fixed male member 22 and the other 23a, 23b or 23c on the rotating female member 23.
This second embodiment enables a minimum variation of the extrusion clearance. It is consequently necessary to find a good equilibrium between the male and female members under pressure in order to limit the axial deformations of the seals. Consequently, the configuration of the seal requires a particular design, notably the presence of annular protuberances as described above, in order to ensure optimum equilibrium: this design consequently requires a high level of engineering and thus results in complex, and indeed sometimes problematic, manufacture and assembly. Furthermore, in this second embodiment, the extrusion and creep clearances are different in each seal. In fact, devices such as rotating joints have different mechanical tolerances depending on whether the seal is located at the top or bottom of the device. Variations in tolerances must be taken into account when designing the rotating joint, which often causes an increase in the costs of such a rotating joint. Consequently, a rotating joint according to the second embodiment is usually used for large diameters and under high pressure.
Furthermore, a rotating joint 21 according to the second embodiment is often associated with an oil barrier system. This system is based on the following principle: an oil barrier is artificially created between two or more seals in order to prevent any leakage of the fluid transferred in the rotating joint. Cavities 27a, 27b and 27c where the seals 24a, 24b and 24c are located are filled with the aid of an insulation fluid by means of two conduits 28a and 28b: the said insulation fluid is placed under a pressure P1 greater than that of the transferred fluid to be sealed, thus ensuring a better contact between the seal and the two contact surfaces of the cavity. This is to prevent any escape of the transferred fluid. Nevertheless, for safety reasons, the insulation fluid is chosen to be compatible with the fluid to be sealed so that should a leak of transferred fluid occur, the insulation fluid would not pollute the transferred fluid. In the example proposed, three seals are arranged on each side of the toroidal chamber as follows: the main seal 24b and the secondary seal 24c are facing in the direction of the toroidal chamber, whereas the insulation seal 24a is facing in the opposite direction. The cavities that accommodate the main 24b and insulation 24a seals are filled with insulation fluid. Should a seal fault occur such as, by way of a non-limiting example, a leakage of insulation fluid, the secondary seal 24c would allow the seal to function until a repair was made.
Although these two embodiments have been widely used for a certain number of years, they have a certain number of drawbacks that have serious consequences for the seal.
Firstly, the 3-race roller bearing poses a few problems. As a reminder, the 3-race roller is used to keep the male and female members concentric. When a pressure is applied, the male and female members deform: the radial clearance within the bearing increases considerably. When external loads, coming from the conduits for example, act on the female member, all of the increases in clearance accumulate on one side of the female member. Radial deformations will therefore be present, resulting in a marked radial creep due to internal pressure. Consequently, the male and female members cannot be kept concentric, which requires a variation in the dimensions of the seal cavity.
In the two embodiments of rotating joints, each seal is placed in a cavity located in the female member and rotates on a surface of the male member. Sealing is thus created on two contact surfaces, one surface on each male or female member, respectively. This type of configuration creates the following stresses on the heel of the said seal:                a circumferential tension, due to the deformation of the rotating joint under pressure;        a tension or compression of the heel in a radial direction, due to the variation in size of the cavity;        in the context of double lip seals, the friction of the first lip on the female member and the friction of the second lip on the male member create circumferential shearing due to oscillating movements. Similarly, the same shearing effect can occur on a toroidal-swivel joint.The addition of all of these stresses can cause irreversible damage to the seal, which, over time, can result in the rupture or failure of the seal.        